Carrier HVAC fan runs- no heat

I have a Carrier A/C that the furnace flame runs until it over heat and the OT switch opens. Then the fan runs on low speed. I replaced the furnace control pcb with no luck.
The LIM-1 and LIM-2 has zero ohms until the OT switch opens. I believe that this tells me that the roll out switch is OK. There is 26 VAC going to the thermostat. The 24 VAC transformer is OK. The fuse is OK. Any clues?

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Carrier HVAC fan runs- no heat

Quote:

Originally Posted by bullsephous

I have a Carrier A/C that the furnace flame runs until it over heat and the OT switch opens. Then the fan runs on low speed. I replaced the furnace control pcb with no luck.
The LIM-1 and LIM-2 has zero ohms until the OT switch opens. I believe that this tells me that the roll out switch is OK. There is 26 VAC going to the thermostat. The 24 VAC transformer is OK. The fuse is OK. Any clues?

ok, your post is very confusing, you start of by saying that your a/c furnace is flaming?

lets try this............

the proper sequence of operation is as follows:
1. Tstat call for heat
2. inducer motor starts and closes pressure switch
3. after a short pre purge igntor will be energized
4. gas valve is energized and delivers gas to manifold and burners
5. gas ignites
6. flame sensor senses flame (must happen within 4 seconds)
7. main blower is energized
8. tstat is satisfied and inducer is de-energized
9. main blower de-energized

Carrier HVAC fan runs- no heat

if the gas shuts off and then the blower energizes is the draft inducer staying on? I'd be willing to be you have a pressure issue meaning draft inducer motor either failing or blocked or the pressure switch failing or blocked.

With the use of gator clips: pull back the wires (but do not pull them off completely, just back a bit) on the spade terminals of the pressure switch. Connect one side of the gator clip wire to one spade of pressure switch, in front of the wire of said spade. Turn heat on. Let the draft inducer run and as soon as the ignitor ignites or glows tap onto the other spade of the pressure switch.

I'll bet it runs continuously then. if it does than stick a friggin sraightened paper clip into the port of the draft inducer and the into the port of the pressure switch, port being where the tube connects on each.

Clear both ports completely. Heck, if you don't have gator clips then just clean the ports and then check heating, turn it on heat. Make sure the tube is back in place first.

Carrier HVAC fan runs- no heat

Carrier HVAC fan runs- no heat

Draft inducing motor? I do not see anything that fits that bill. There is a flame roll out sensor, but I believe that it is OK since if any one of the three (OT sensor, thermal fuse & the rollout switch) are shorted. If any one is open, then the fan runs all the time and the flame never starts. Enlighten me, PLEASE.

Carrier HVAC fan runs- no heat

Carrier HVAC fan runs- no heat

Thanks for the diagram. The old board had built in jumpers- 1 between Gas 1 & Gas 3. The other between GH & R. That was the whole problem. Mostly due to my trifocals and the old pcb. Thanks to all who have helped me !

Carrier HVAC fan runs- no heat

Quote:

Originally Posted by bullsephous

Draft inducing motor? I do not see anything that fits that bill. There is a flame roll out sensor, but I believe that it is OK since if any one of the three (OT sensor, thermal fuse & the rollout switch) are shorted. If any one is open, then the fan runs all the time and the flame never starts. Enlighten me, PLEASE.

That's why you post make, model and serial number first!
I had one the other day that was doing the exact same thing as yours but it had a draft inducer. I jumped across the pressure switch and it worked so I assumed on yours...